stretchy jewelry cord
Measure your ankle & cut that amount of cord PLUS a few extra inches for tying off the ends.
Now, string on your beads. I find it much easier to work on a piece of felt. The beads don't roll around. Once you're sure you've got the correct amount of beads strung, tie a knot. (Pull nice & tight. You don't want any saggy beads floating around.) Then dab the knot with super glue. I tie off two more times & dab with another dollop of glue. Just to be extra super-duper sure it won't come undone. Then cut off the excess cord.
(And of course this same process applies for bracelets!)